As is known, the aircraft industry employs nitrided
Or carburized steel components, which are first carburized, then shot peened, and finally ground.
Shot peening is a mechanical process of cold working the component surface to produce an isotropic residual compression state in the outermost layers of the component.
Grinding often affects the final surface condition of shot peened components in terms of properties such as residual stress, microstructure, and consequent resistance to wear and fatigue.
In particular, localized immission of energy, in the form of heat produced in the grinding area, can cause serious thermal damage in the surface layer, if grinding parameters are not carefully controlled.
If the temperature of the component exceeds tempering temperature, grinding may produce microstructural martensitic changes at metallurgical level, resulting in so-called overtempered martensite: a metastable phase involving softer surface regions and a general decline in mechanical properties.
As a result, overtempered martensite may accelerate the onset of cracks; and grinding may also alter the residual stress pattern of the material.
In the worst case scenario, the material may present residual compressive stress in the surface region, and residual tensile stress in an inner region close to the surface, thus resulting in abrupt changes in stress of the material.
The ultimate amplitude and uniformity of the stress may vary considerably.
A need is therefore felt within the industry to determine the effect of grinding on the stress of components, to enable an accurate evaluation of the component's resistance to fatigue.
Known chemical etching component inspection methods are only effective for heavy microstructural steels, and invariably involve a certain amount of subjective evaluation.
Moreover, they also fail to determine changes, caused by overtempering, in the residual stress of carburized or nitrided steel components.
An alternative method of determining stress of a component employs the Barkhausen effect.
According to the Barkhausen effect, the magnetic flux inside a component of magnetizable, e.g. ferromagnetic, material exposed to a varying magnetic field does not vary continuously, but goes through discrete changes, which induce, in a coil placed close to the component, voltage pulses that can be amplified and connected to a loudspeaker to generate acoustic pulses known as Barkhausen noise.
The discrete changes in the magnetic flux inside the component are related to discontinuous movements of the magnetic domain edges. More specifically, in a nonmagnetized component, the magnetic domains are oriented randomly, so mean magnetization of the body is zero. When exposed to an external magnetic field, the orientation of the component's domains tends to change accordingly, and goes through the movement of the adjacent domain walls, so ‘macroscopic’ magnetization of the body goes through discrete changes owing to the magnetic discontinuities within the component.
Barkhausen noise characteristics are known to be affected by tensile or compressive stress of the component.
A need is felt within the industry for a system and method for determining stress of a component made of magnetizable material using the Barkhausen effect, and which are easy to implement.
A need is also felt for a straightforward, easy-to-implement system and method for determining said stress at different component depths.